1. Field of the invention
The present invention relates to an LCD (liquid crystal display) projector to project an enlarged picture of an image displayed on an LCD to a projection screen.
2. Description of the related art
LCD projectors have been of wide interest because of their compactness and ease of installation. FIG. 1 shows an example of a prior art LCD projector. The LCD projector is basically made up of light source 51, LCD element (hereafter, referred to as LCD) 52 and projection lens 55. In the case that a twisted-nematic (TN) mode LCD is used as LCD element 52, polarizer 53 and analyzer 54 are arranged in front of and behind LCD 52, respectively. The arrangement is shown as TN-LCD 50 in FIG. 1. Light source 51 is composed of a parabolic mirror and a lamp located at the focus of the mirror, thereby emitting a light beam parallel to the optical axis of the parabolic mirror. The beam will be referred to-as projection beam 57 below. The component of projection beam 57 polarized parallel to the optic axis of polarizer 53, when it is incident on polarizer 53, undergoes intensity modulation corresponding to the display image generated on LCD 52. The major part of the perpendicularly polarized component, however, is absorbed by polarizer 53, thereby causing heat generation.
It has been common, in order to obtain a projected picture of high contrast and high quality, that LCD 52 is made of an active matrix of pixels, each provided with a switching element. While LCD 52 has the advantage of allowing a large-sized projection picture to be readily obtained with a small-sized LCD, a problem is that the switching elements and opaque electrode films provided in the active matrix prevent transmission of projection beam 57 through LCD 52, which deteriorates the light-utilization efficiency of the LCD projector, thus causing the projected picture to be dark or less luminant.
While the failure in luminance of the projected picture described above may be remedied by using a bright light source, the use of a bright light source causes another problem when the TN-LCD is used: since the perpendicular component of the projection beam (the light component polarized perpendicular to the optic axis of polarizer 53) is absorbed by polarizer 53 causing heat generation which increases as the light source is brighter, the bright light source causes deterioration of the polarizer due to a temperature rise caused by heat generation.
In order to solve this problem, S. Shikama et al. presented a novel polarization beam-splitting optical device, published in Proceedings of Eurodisplay 1990, Report of The Tenth International Display Research Conference, pages 64 through 67. FIG. 2 shows the optical layout of the Shikama et al. optical device. The optical device is made up of polarization beam splitter (PBS) 61, mirror 62 and halfwave plate 63. PBS 61 has polarization beam-splitting plane (PBSP) 64, which is oriented with respect to projection beam 57 so that the plane of incidence is parallel to the optic axis of polarizer 53. Thus the p-polarized component of projection beam 57 transmitted through PBSP 64 passes through polarizer 53. Mirror 62 is disposed on the optical path of the s-polarized component reflected on PBSP 64 and is oriented so that the s-polarized component is deflected toward the predetermined region of the incidence surface of polarizer 53. Halfwave plate 63 is disposed in the optical path of the s-polarized component reflected on mirror 62 with its optic axis making an angle of 45.degree. with the direction of polarization of the s-polarized component. Thus the s-polarized component is optically rotated to be a p-polarized light, which is then transmitted through polarizer 53. The advantages of the Shikama et al. optical device are that, since approximately 99% of projection beam 57 is delivered from their polarization beam-splitting optical device as a p-polarized beam, the brightness of the projected image is markedly improved, and that, since the s-polarized component impinged on polarizer 53 is only a small part of projection beam 57, heat generation in the polarizer due to absorption of the perpendicular component becomes far less, which allows a high intensity light source to be used without any substantial temperature rise.
While the Shikama et al. optical device provides a way of obtaining a bright projection image, a further problem has been left unsolved related to the illuminance distribution on the incidence surface of LCD element 52. FIG. 3 shows diagrams for illustrating the manner of illuminating the incidence surface of TN-LCD 50 in the LCD projector shown in FIG. 1: FIG. 3(A) is a plan view illustrating the optical path of projection beam 57, FIG. 3(B) illustrates the light-utilization efficiency for an oblong incidence surface, and FIG. 3(C) shows the illuminance curve on the same surface as FIG. 3(B).
Projection beam 57 emitted from light source 51 is perpendicularly incident on the incidence surface of TN-LCD 50 with an optical path parallel to the axis of the parabolic mirror. Since projection beam 57 has a circular cross section, the part of projection beam 57 surplus to the incidence surface (hatched with oblique lines in FIG. 3(B)) is unutilized to illuminate LCD 52. This unutilized part of projection beam 57 increases as the shape of the incidence surface differs from that of the incident-beam cross section on the incidence surface. Hereafter, the former and latter shapes will be referred to as the incidence-surface shape and the incident-beam shape, respectively. In FIG. 3(B), the incidence- surface shape is an oblong rectangle and the incident-beam shape is a circle.
It has been widely known that, when an image is projected on a large projection screen, use of an oblong or wide screen as used in a movie is preferable, because this wide projection do not cause watchers to be fatigued and serves to make a forceful impression on the watchers as well as to appeal to their emotions. This is the case in a television image plane. Specifically, a high-definition television receiver, which is capable of producing a high-quality and high-resolution image, has an image plane of an aspect ratio of 9 : 16, more oblong than the aspect ratio 3 : 4 adopted in an ordinary television receiver.
When wide projection is desired in an LCD projector, an LCD element with an oblong rectangular image plane is employed. In this case, projection beam 57 has to have a cross-sectional diameter equal to or larger than that of the circumscribed circle of the incidence surface in order that the whole incidence surface is illuminated, as shown in FIG. 3(B). Since the part of the projection beam outside the oblong rectangular incidence surface, i.e., the part unutilized for image projection, increases as the aspect ratio of the incidence surface deviates from 1, light utilization efficiency decreases, this an average illuminance of the incidence surface decreases as the incidence surface becomes more oblong. Further, since the circumscribed circle diameter for a constant area of the incidence surface increases as the aspect ratio deviates from 1, the difference between the illuminances in the central and side regions of the incidence surface tends to be more prominent, as the incidence plane is more oblong, as is seen from FIG. 3(C). This difference in illuminance causes a difference in luminance of the projected picture, further causing the quality of the projected picture to be lowered.